Since microwaves have the potential to heat both peripheral and deep tissues, chronic exposure to low energy levels (below 10 mW/cm2) may have consequences for the ability of warm-blooded species to regulate their body temperature. In particular, the normal development of thermoregulatory response systems may be compromised if such exposure occurs in utero or during the first few weeks after birth, when homeothermy is not yet established. The major aim of this study is to determine if a critical period exists in the growth of the immature animal when microwave exposure at selected ambient temperatures can significantly alter physiological and/or behavioral thermoregulatory abilities. Pregnant rats will receive daily (8h/day) microwave (1 or 5 mW/cm squared) or sham exposures to 2450 MHz continuous microwaves at controlled ambient temperatures of 25 or 35 C from day 1 to day 19 of pregnancy. Neonates from dams in each treatment group will be microwave or sham exposed to the same conditions from 7 to 19 days of age. Physiological tests, conducted at 4-6 days and 20-22 days of age, will determine if pre- or postnatal microwave exposures have altered thermoregulatory ability. These tests will include measurements of oxygen consumption, rectal and tail-skin temperatures, to determine if thermal adaptation has occurred during the treatment period and to assess the additional effect(s) of microwave exposure. In addition, rats from each treatment group will be tested in a thermal gradient at 14 and 22 days of age to determine if the normal thermopreferendum has been altered by the exposure treatments. Observed effects will be related to changes in thermoregulatory responses measured during acute exposure to controlled microwave fields (1, 5, and 10 mW/cm2) in representative age groups and also to detailed disometric measures. Evidence for unusual changes in thermoregulatory capability attributable to microwave exposure during a critical stage of development will generate concern that human infants should be protected from exposure to low intensity microwave fields; alternatively, evidence for enhanced thermoregulatory capability may indicate the feasibility of utilizing microwaves for the incubation of immature humans and other mammalian species.